Why no negative Z in "Focus Test "

Happy New Year to all.
New user first post. I have been using the software for a month now and I am very pleased and impressed with it. A very good value for the money. ( I bought it last week )
My machine is a DIY, I am running GRBL on a CNC shield. 3-axis with a 250mm sq. envelope and about 50mm in Z.
My question is why won’t “Focus Test” accept negative numbers ?
I come from the machine tool world where everything is “negative” from the “homed” machine coordinates. So my standard procedure would be to set zero at the top of the work.
So in my head I would go to what I think is about the right focus, take Z up a little bit, and do the Z focus test starting at zero and go to Z-3mm or so.
I imagine I could start below zero and work up but that really seems backwards to me.
I have read some other posts about Ruida controllers not allowing negative numbers but GRBL does and lightburn is OK with that , just not the focus test.
Any help will be greatly appreciated.


Mostly because it’s not valid for DSP controllers, and it’s not required to make it work for GCode controllers.

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Hi Oz
Thanks for your reply.
I thought that there were 2 different versions of LB for DSP and GCode . So couldn’t a negative Z be implemented in the GCode version of focus test?
So what is the accepted way to “Make it work” on a GCode machine ?

Thanks again


The “Reverse Z direction” switch in Edit > Device Settings negates all the Z values output for machines that require it.

It’s not impossible to do, it’s just so simple to do a focus test in other ways (IE : ramp test) that I honestly didn’t expect it to be nearly as popular as it seems to be, or as heavily argued about how it should work. :slight_smile:

Does the documentation have any instructions for running the focus test? I checked the doc, but I might have missed it

I don’t think so, no.

Hi Oz
Thanks again for your reply.
I have to admit that I am a total noob when it comes to lasers. This is my first foray into getting one properly setup. I just figured that if the software came with a focus test , that was what I am supposed to use. I did not mean to ruffle any feathers :slight_smile:
I just watched a couple of videos on a ramp test and will go try that out.


I won’t bug you any more about the "Focus Test "
Tonight I played with the Ramp test and works really well :grinning: I will use that from now on.

I used a piece of 1.5mm ply at about a 15-20 deg. angle and adjusted the speed at 100% power until I had a short perforation which was more precise than guessing where the line was thinnest. ( old eyes )

Thanks again for all your help


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